1. Field of the Invention
The invention relates in general to the field of integrated circuit manufacturing technology and, more particularly, to a new material which can be deposited as a layer onto a semiconductor wafer to serve numerous purposes, including acting as an interdiffusion barrier or an adhesion promoter.
2. Description of the Related Art
In the manufacturing of integrated circuits, which are also referred to as semiconductor devices, numerous microelectronic circuits are simultaneously manufactured on semiconductor substrates. These substrates are referred to as wafers. A typical semiconductor wafer is comprised of a number of die. Each die contains at least one microelectronic circuit, which is typically replicated across all of a wafer's dies. One example of a microelectronic circuit which can be fabricated in this way is a dynamic random access memory or "DRAM".
Although referred to as semiconductor devices, integrated circuits are in fact fabricated from and contain numerous materials of varying electrical properties. These include insulators or dielectrics, such as silicon dioxide, and conductors, such as aluminum or tungsten, in addition to semiconductors, such as silicon and germanium. The most common semiconductor employed is silicon.
In state of the art integrated circuits, it is common for the design to require interfaces between layers of varying electrical properties. The interface between the two layers may constitute the entire surface of the die, or the interface may occur when conductive paths or openings are formed to connect or complete different circuits that have been fabricated within a die. One method to complete different circuits is through the use of conductive paths in or on an insulative layer which provide for an interface between a conductive, typically metal, layer and a semiconductive substrate, typically a silicon layer. Conductive paths of this variety are typically referred to as contact openings or contacts. The contact opening allows for an interface between the underlying semiconductive layer and the subsequently deposited conductive layer.
It is common for a design to require an interface between other layers, such as between two different layers of metallic conductors, between an insulator and a metal conductor, between an oxide and a semiconductor, and between two different semiconductors. Interfaces between other layers are also known. Again, these interfaces may constitute the entire surface area of a die or they may be restricted to the narrow recesses of a conductive path formed between the two layers. Conductive paths providing an interface between pairs of layers may be referred to in the art as contacts or contact openings, or they may be referred to as vias or some other term. Because of the inconsistent use of these terms, the term conductive path will be used herein to refer to such openings formed in a semiconductor process to connect layers regardless of which layers are thereby connected.
There are several difficulties inherent to these interfaces. One difficulty is due to the existence of common fabrication steps which require protracted annealing of the devices at elevated temperatures, often in excess of 500.degree. C. These temperatures are especially problematic for interfaces between semiconductor and conductor. At these temperatures the metallic conductor and the semiconductor can rapidly interdiffuse into the adjacent region. The interdiffusion of these materials is driven by the concentration gradients. This leads to a lower energy state of the composite material, which is the preferred stable state. This desire or trend toward a lower energy state is embodied in the Second Law of Thermodynamics which states, in effect, that whenever a spontaneous event takes place in the universe, it is accompanied by an overall increase in the degree of randomness (i.e., an increase in entropy). As a practical consequence of the Second Law of Thermodynamics, matter has a tendency to diffuse from areas of high concentration (i.e., high energy) to low concentration (i.e., low energy); hence, the interdiffusion of conductive and semiconductive layers. The interdiffusion changes the electrical properties of the two regions and in particular the semiconductor region, resulting in an increased likelihood of the production of inoperative devices. The interdiffusion of the two regions can also occur at room temperature, although at a much slower diffusion rate. Interdiffusion between other layers may also occur, increasing the likelihood of producing inoperable devices.
These interdiffusion concerns generally have dictated the need for a barrier material to be deposited at the interface of the two layers when there is sufficient concern over interdiffusion of the two regions. Specifically the barrier material is deposited onto the surface of one layer prior to the deposition of another layer onto the barrier layer. For example, a barrier material can be deposited into a contact opening to prevent the interdiffusion between an underlying semiconductor layer and a subsequently deposited conductor layer. Titanium nitride (TiN) and titanium tungsten (TiW) are typical of the compounds used as barrier materials (i.e., to form a barrier layer). The barrier materials are typically deposited as a thin film or layer over the exposed surface of the die, including any intended interface. The deposited surfaces would include the walls and base of conductive paths. The thickness of these barrier films is typically in the range of 200 .ANG. to 1000 .ANG. although the use of films of other thicknesses is known.
A second difficulty associated with these interfaces is a corollary to the first. Despite interdiffusion concerns, good contact between successive layers of the integrated circuit should be maintained. Therefore, any layer employed as a barrier layer to minimize interdiffusion should exhibit good adhesion qualities to the adjacent layers. Indeed, a thin film may be deposited strictly because of its adhesive properties when the two adjacent layers do not readily adhere to each other. Finally, barrier layers can also be used to tailor the electrical properties of contacts.
One difficulty that limits the widespread use of certain materials as barrier and adhesive layers is their effectiveness at higher temperatures. Although generally effective at room temperature, certain materials may lose their barrier or adhesive properties at the high temperatures to which semiconductors are necessarily exposed. Barrier layers composed of TiN and TiW generally suffer from this limitation. Elevated temperatures are not only common for annealing steps but are often dictated by the limitations of the deposition techniques used to deposit materials onto the semiconductor wafer. Thus, a barrier material which does not exhibit a loss in either its barrier or adhesive properties at elevated temperatures would be extremely useful.
A further complication in integrated circuit manufacturing is the ever increasing trend of reducing the size of the microelectronic circuits. As the size of these circuits, and therefore the size of die regions, decreases, the number of devices produced from any one wafer increases dramatically. However, as the size of these devices decreases, the percentage of reliable circuits produced on any one wafer becomes highly dependent on the ability to deposit films, including films deposited for barrier and adhesive purposes, uniformly across surfaces. These size reductions dictate that barrier materials be deposited at ever decreasing thicknesses, which means that these barriers are even more susceptible to elevated temperatures and the disastrous effects these temperatures can have on interdiffusion. Therefore, one requirement for any new barrier material, deposited as a barrier film or layer, would be that it exhibit greater inherent resistance to thermal energy than those materials previously employed. Additionally, it would be preferred that the adhesion properties of new barrier materials be at least as good as those previously employed. Finally, the trend in size reduction also dictates that new barrier materials must yield deposited films exhibiting greater uniformity in thickness (i.e. they must exhibit improved step coverage) than previous materials. The goal is to avoid the deposition of a barrier film which has holes or gaps within it. If present, these holes or gaps would tend to reduce the barrier properties of the film. As a corollary to this requirement, it would be preferable that any new material used as a barrier or adhesive layer could be deposited by those techniques currently used to deposit thin uniform films. Sputter deposition, chemical vapor deposition, and plasma enhanced chemical vapor deposition are the techniques most commonly employed to deposit the thin films of interest.